The metastable zone width (MSZW) of SrCl2·6H2O during cooling crystallization was determined by the focused beam reflectance measurement (FBRM). The effects of the saturation temperature, cooling rate, stirring rate, Ca2+ ion impurity, and seed crystals on the MSZW were investigated. The MSZW increased with the increase in the cooling rate and decreased with the increase in the saturation temperature, stirring rate, and Ca2+ ion concentration. The seeded MSZW decreased than the unseeded one, and seed surface is the key factor for nucleation. Small seeds provide a large surface area with high adsorption capacity for solute. They can consume more supersaturation on seed growth, thus retraining nucleation.